Moved to https://github.com/bitcoin-core/bitcoin-maintainer-tools/blob/master/optimize-pngs.py
Bitcoin Core should focus on the full node implementation, not on scripts to compress png images.
This script is only used when new PNG files are added to the repo. This happens about once every two years. So fetching the script from the other repo should not be a burden, but removing it from this repo is going to cut down on the meta files we need to maintain in the main repo.